The present invention relates to a system and method for testing combustible gas leaks through valves, and in particular, to test for leaks through gas control valves on a "double block and bleed" arrangement.
Various arrangements for gas piping have been developed to promote safe operation of burners, and to prevent explosions in the event of a leak in a safety shutoff valve used to prevent gas from reaching the burner when it is not in use. The most common of these arrangements is commonly referred to as the "double block and bleed" arrangement. In its most basic form, the arrangement requires two automatic (motor or solenoid-operated) safety shutoff valves mounted in series along the gas supply line in order to provide redundant shutoff capability along the gas line. These valves are designed as "fail-closed." In other words, in the event of valve failure, the valve will close and prevent gas from passing therethrough.
In addition, a bleed line is connected to the gas line between these two valves and is vented to atmosphere through another automatic valve, commonly referred to as the vent valve, which is designed as "fail-open", i.e. in the event of failure, the valve will automatically open and allow any gas in the bleed line to escape. The bleed line and the vent valve serve to remove any gas which might leak through the first safety shutoff valve into the gas supply line between the first and second safety shutoff valves.
Downstream of each safety shutoff valve, a valved test port is usually provided to check for leakage through the safety shutoff valves while the burner is off. In some cases provision is made to energize/close the vent valve (or a manual locked valve ahead of the vent valve) to assist in the testing procedure. A manual shutoff cock is often installed downstream of both safety shutoff valves in order to facilitate the testing. While many systems presently available have recognized the need to test the safety shutoff valves and have provided methods for doing so, there are presently few, if any, systems which test for leakage through the vent valve.
If the vent valve leaks, gas escapes through the bleed line to atmosphere while the burner is on. While such gas loss will usually not present a significant safety hazard, potential fuel gas loss through a defective vent valve is significant enough to justify the use of a leak test system to minimize the loss of gas. This is especially so if the system is simple to use and, as an added benefit, provides additional methods for detecting leakage through the safety shutoff valves, thus providing a backup leak test system for the safety shutoff valves. Therefore, there is a need for a leak test system which will test for leakage occurring through the vent valve, in addition to being able to test for leakage through the safety shutoff valves positioned in series along the gas supply line.